Oil reservoirs are being discovered below the sea bed. In several cases, these reservoirs are located at considerable depths, either below deep water or deep below the sea bed or both. Thus access to these reservoirs for the purpose of recovering oil requires use of complex, expensive above-sea oil platforms to locate the reservoirs and to extract oil therefrom.
Oil can be recovered from sub-sea reservoirs using the pressure naturally available at the reservoir. Additional oil can be recovered by injection of seawater at high pressure into the reservoir, thus forcing out additional oil. However, the oxygen dissolved in seawater, the amount of which depends on temperature, causes corrosion of equipment such as the water injection line. Further, a high concentration of oxygen degrades the quality of oil in the reservoir. Thus it is undesirable for dissolved oxygen to be present at naturally occurring levels in seawater injected into the reservoir.
Several approaches have been taken to reduce the amount of oxygen dissolved in seawater, in particular with the view to injection of the deoxygenated seawater into sub-sea oil reservoirs.
Fuel gas stripping using a conventional counter-current gas-liquid contacter is a convenient method for reducing oxygen dissolved in seawater. However, this method requires a large tower and the apparatus has a large footprint. Thus this method is inefficient for use at off-shore platforms where space is at a premium.
Vacuum towers are used for separation of dissolved gases from liquids. Again, this method requires use of at least one large tower. Further, the method has high operating costs.
Oxygen scavengers injected into seawater react with the dissolved oxygen and thereby reduce the oxygen content. However, the chemicals used as scavengers are expensive, and significantly affect the cost of operating a seawater injection system.
Some commercial designs of apparatus for regenerative nitrogen stripping of oxygen from seawater, such as that from Minox described below, may allow the use of less space than that for either fuel gas stripping or vacuum tower methods, and the method incurs lower costs than use of oxygen scavenging compounds. However, use of the method requires incorporation of apparatus and processes for remediation of operating problems that can arise including, for example, foaming and entrainment that cause fouling of the down stream equipment, and so affect performance of the overall process.
Exemplary processes are described in the following patents.
Lydersen in U.S. Pat. No. 4,565,634 (1986) describes use of a vacuum tower to separate dissolved oxygen from seawater. In essence, dissolved gases are desorbed from the seawater under reduced pressure. As shown in the figure of this patent, nitrogen flows downward through the tower cocurrently with the water, acting as a stripping gas, and a gas stream drawn from the bottom of one stage are pressurized to the pressure of the previous stage and reinjected. The net effect is reduction of oxygen content of the seawater drawn from the bottom of the apparatus to about 0.04 ppm (40 ppb). Here and throughout all such values (percent, ppm, ppb) are expressed by weight.
Bland and Palmer in U.S. Pat. No. 4,612,021 (1986) describe a process for reduction of an unwanted gas in a liquid by contacting with another gas. In essence, the second gas serves as a stripping gas, for example for reduction of oxygen in seawater by supplying nitrogen as said seawater is injected into the main gas ejector. In an auxiliary gas ejector, positioned above the main gas ejector in the figure illustrating the apparatus of this patent, dissolution of entrained nitrogen displaces oxygen dissolved in seawater, which is then reacted in a catalytic burner. Make up nitrogen is then provided. The oxygen dissolved in seawater is reduced to about 0.25 ppm (250 ppb).
Henriksen in U.S. Pat. No. 4,752,306 (1988) describes a system in which an inert stripping gas is pumped in turbulent concurrent flow with seawater to remove dissolved oxygen. The resulting gas mixture and liquid are separated, the gas mixture is then treated to remove oxygen, and the stripping gas is then returned to treat more seawater. In this manner, the oxygen dissolved in seawater is reduced to about 0.1 ppm (100 ppb). Again, when nitrogen is the inert stripping gas the efficiency and cost of the process depend on the purity of nitrogen. A distinguishing feature of this process is that nitrogen is purified after use by removal of oxygen in a separate catalytic reaction chamber, identified by numeral 20 in FIG. 1 of '306, before recycling through the process.
Mandrin and Keller in U.S. Pat. No. 5,006,133 (1991) describe a method and apparatus in which a fuel including natural gas is used as both a stripping gas to remove oxygen from seawater and a fuel to be oxidized by said stripped oxygen in a catalytic process. A delivery means is provided for recycling the oxygen depleted stripping gas to the deoxygenator thus reducing the oxygen content of seawater. It is notable that methane in natural gas is not easily oxidized and so methanol or hydrogen is used to initiate the catalytic combustion part of the process. The oxygen dissolved in seawater is reduced thereby to about 10 ppb.
Norinco Co., an Indian firm, disclosed a Minox™ deoxygenation system comprising two separators and a catalytic reactor for reducing the oxygen content of seawater from about 9.1 ppm to about 5.2 ppb. The apparatus is compact and can be used at hazardous and non-hazardous locations through use of suitable enclosures. The Minox™ deoxygenation process comprises a series of stages. Methanol is added to the effluent of the first stage stripping gas stream, and the mixture of gases is heated over a catalyst so that methanol reacts with oxygen to form carbon dioxide and water. The deoxygenated stripping gas is then returned to a second stage for further use. A problem that can occur with the recycle systems of this type is that the catalyst and downstream equipment are prone to fouling from entrained saltwater.